If President Trump executes his plans to create a U.S. Space Force, there will be repercussions, promise Russian diplomats and politicians.

The president declared Monday that the United States must return to outpacing China and Russia as competitors in space exploration and directed Pentagon leadership to create a a sixth, space-oriented branch within the U.S. military.

Russia warns Trump it will retaliate if the president proceeds to build a U.S. Space Force, arguing the development of...